1. Field of the Invention
This invention relates to a method of accurately reading out a radiation image at high speeds from a stimulable phosphor carrying the radiation image stored therein, and an apparatus for carrying out the method.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted from the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to a radiation passing through an object to have a radiation image stored therein, and is then scanned with stimulating rays such as a laser beam which cause it to emit light in the pattern of the stored image. The light emitted from the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted to an electric image signal, which is processed as desired to reproduce a visible image on a recording medium such as a photographic light-sensitive material or on a display device such as a cathode ray tube (CRT).
This radiation image system using the stimulable phosphor sheet is advantageous over conventional radiography using a silver halide photographic material in that the image can be recorded over a very wide range (latitude) of radiation exposure and further in that the electric signal used for reproducing the visible image can be freely processed to improve the image quality for viewing, particularly for diagnostic purposes. In more detail, since the amount of light emitted upon stimulation after the radiation energy is stored in the phosphor varies over a very wide range in proportion to the amount of energy stored therein, it is possible to obtain an image having desired density regardless of the amount of exposure of the phosphor to the radiation by reading out the emitted light with an appropriate read-out gain and converting it to an electric signal to reproduce a visible image on a recording medium or a display device. The electric signal may further be processed as desired to obtain a radiation image suitable for viewing, particularly for diagnostic purposes. This is very advantageous in practical use.
In the aforesaid radiation image recording and reproducing system, the read-out step for reading out the radiation image from the stimulable phosphor sheet carrying the radiation image stored therein is typically conducted by two-dimensionally scanning the stimulable phosphor sheet with a light beam such as a laser beam which causes it to emit light in proportion to the stored energy of the radiation, and sequentially detecting the emitted light and converting it into an image signal by use of a photodetector such as a photomultiplier.
As described above, when the stimulable phosphor carrying the radiation energy stored therein is exposed to stimulating rays, the stimulable phosphor releases the stored radiation energy as light emission. The intensity of the light emission rises to near maximum level quickly (for example, in several nanoseconds) after the exposure of the stimulable phosphor to the stimulating rays is started, and thereafter decreases gradually. Even after the exposure to the stimulating rays is finished, the light emission continues as after-glow for a period corresponding to the response time intrinsic to the stimulable phosphor. Therefore, when the stimulable phosphor sheet is scanned with the stimulating rays and the light sequentially emitted from the stimulable phosphor sheet is photoelectrically detected, not only the light component emitted from the picture element currently being exposed to the stimulating rays but also the after-glow components emitted from the picture elements which have already been exposed to the stimulating rays are detected as the radiation image component of the picture element currently being exposed to the stimulating rays. As a result, discrimination of the signals among the picture elements is not attained completely, and the sharpness of the visible image reproduced by use of the signals detected as described above becomes low. In the detection of the intensities of light emitted from a plurality of picture elements of a radiation image, the resolution among the picture elements is decreased as the scanning speed of the stimulating rays is increased and as the response time of the stimulable phosphor becomes longer. Accordingly, in the case where a stimulable phosphor exhibiting slow response to stimulation (i.e. a stimulable phosphor exhibiting long after-glow) is used or the scanning speed of the stimulating rays is increased, the sharpness of the image becomes low and it becomes impossible to obtain a visible radiation image having a high image sharpness satisfactory for practical use.
Since there is desired a read-out apparatus which can quickly process many stimulable phosphor sheets, that is, a read-out apparatus having a high read-out speed (scanning speed), and since there is substantively a limit to the shortening of the response time of the stimulable phosphor that can realized by improvement of the stimulable phosphor, a need exists for a method of eliminating the lowering of the sharpness of the radiation image which is essentially attributable to the response characteristics of the stimulable phosphor.